As pointed out in application Ser. No. 838,575, electrical discharge machining, hereinafter referred to as EDM, makes use of electroerosive power pulses applied between a workpiece and a tool electrode spacedly juxtaposed across an electrode gap flooded with a dielectric coolant which also serves to carry away the detritus of the electrical discharge machining process.
The tool electrode is generally formed with the desired configuration of the cavity or shape complementarily desired in the workpiece. A train of power pulses is then formed to create localized material-removal discharges having a tendency to produce cumulatively overlapping craters in the workpiece surface; the total surface juxtaposed with the tool electrode is thus machined uniformly over the parts thereof confronting the tool electrode and receive a configuration conforming to the shape of the tool electrode.
During the machining operation, small metallic or conductive chips or particles are carried away by the liquid dielectric which floods the gap and is generally circulated therethrough while the tool electrode is advanced relative to the workpiece by a servomechanism designed to maintain a predetermined gap spacing or to approach the desired gap spacing as accurately as possible.
In the aforementioned application Ser. No. 838,575, it is pointed out that such machining techniques contain many elements which interfere with the predictability of a gap condition and make it difficult to achieve the desired machining state for each pulse. In that application, there is described an "adaptive" power supply in which the character of the pulses is adjusted automatically to suit the gap conditions. In accordance with the principles of that application, electroerosive discharge pulses are optimized to adjust the pulse frequence and other essential pulse parameters, including pulse width, interval, etc. to optimize the machining process.
That system includes closed-loop self-adaptive pulse generators for optimal timing control in an on/off or digital sense of an electronic switching element or power switch operatively connecting d.c. machining-power sources with the machining gap to provide self-adaptive material removal spark-producing pulses. The term "closed-loop" generator is used herein to refer to a system in which the machining gap itself is part of the signal-pulse generator so that the gap itself controls the power circuit and the turning on and turning off the power switch in response to the gap state, thereby permitting the power to be delivered to the gap in a precisely regulated discharge and permitting the termination of the discharge by turning off the switch also in response to the gap state.
The power switch is of the solid-state type and can carry a high d.c. power while ensuring rapid and reliable switching operations under heavy load. The "on" and "off" characteristics of such switches can be generally referred to as "instantaneous" although from time to time the response characteristics may not be perfectly vertical. A sensor in the system provides the machining-gap information manifesting the gap state and this information is derived in the form of a continuous signal or analog signal (the electrical signal obtained being the analog of the gap parameter measured) and fed to an integrating network.
The level of the analog signal changes with time and represents one or more gap parameters at any given time such that the rate of change of the analog signal closely reflects the overall gap state as well as the gap state prior to the discharge in question.
The analog signal is fed to a threshold gating circuit, preferably a Schmitt trigger, wherein the integrated analog signal is compared with a threshold reference to produce either of two possible digit states with sharp interstate transition depending upon the magnitude of the analog signal and thereby function to perform an analog-digital conversion. Thus in one state of the circuit is response to the analog signal, no output appears at the Schmitt trigger (corresponding to the digital "0" signal state). The circuit in another condition of the analog signal produces a square-wave output corresponding to the digital "1" state. This pair of signals functions as a trigger to close the power switch and open the latter, respectively, thereby rendering the switch alternately conductive and nonconductive, preferably via intermediate amplifying stage. By thus controlling the point at which the subsequent discharge is to be initiated, the direction of this discharge and the point at which the discharge is to be terminated or in accordance with the prior condition of the gap and of the gap condition at the moment of the new discharge, a truly self-adaptive regulation of the power pulses is obtainable. In the last-mentioned application it was pointed out that a servocontrol system for regulating the machining gap is practically an essential in EDM operations.